Die attach adhesives for use in the microelectronics industry must be capable of being processed quickly and must be thermally or electrically conductive. One group of currently preferred die attach adhesives is the siloxane-modified polyimides. Typically, these are paste-like materials that may or may not contain solvent.
In the fabrication of semiconductor devices, the adhesive is deposited onto the paddle of the lead frame, the die then placed on top of the adhesive and any solvent removed by means of heat. Up to the time of the deposition of the die onto the adhesive, the solvent must be retained by the adhesive paste system for adequate adhesion. After die placement, the solvent must be removed from the adhesive as completely as possible, because if it is retained in the adhesive, it eventually will cause voids when it does evaporate upon exposure to high temperatures. These voids are locales of adhesive failure during thermal cycling. Therefore, removal of the solvent is essential to the performance of the adhesive. Usually, removal is accomplished by baking off-line. This interrupts on-line production and makes the use of adhesive pastes and solutions containing solvent inefficient. In addition, the trend in the industry is toward the increased use of large dice (1".times.1" or larger). Because the bulk of the adhesive is covered by the die, which interferes with the escape of solvent, large dice obviously give greater interference in removal of the solvent from the adhesive.
In addition to the problems that are encountered by the presence of solvent in the adhesives, the addition of thermally conductive fillers to adhesives tends to make the adhesive brittle and even more susceptible to mechanical and thermal stress, particularly when used at high loadings.
Thus, there is a need for a die attach adhesive that does not contain solvent or require baking off-line, and that has sufficient flexibility to accept electrically and thermally conductive fillers. A thermoplastic die attach adhesive in film form that bonds rapidly and that can exert adequate adhesion during the processes of wire bonding and molding, and that can be used at temperatures that do not oxidize lead frames or produce excessive thermal stress, would serve this need.
U.S. Pat. No. 4,395,527 issued to Berger discloses that the properties of adhesives can be improved by the incorporation of siloxane and gives a broad general teaching for the syntheses of siloxane-containing polyimide compositions.
U.S. Pat. No. 4,652,598 issued to Edelman discloses the incorporation of polysiloxanes into polyimides to enable the water of imidization, and any solvent used, to be readily removed without producing significant imperfections in the polyimides.
Although, there if a broad general teaching for the syntheses of polyimide compositions in these patents, there is no recognition of the critical combination of reactants required to obtain the specific die attach adhesive properties of the thermoplastic film of this invention.